Centrifugal pumping devices are widely used throughout industry, particularly in the chemical and petroleum processing fields. The typical centrifugal pump includes an impeller connected to one end of a rotary shaft which is rotatably driven by an electric motor, steam turbine, or any suitable prime mover. The impeller is housed within an impeller casing. Fluid entering the casing at the center of the impeller is radially displaced and pressurized by the rotating impeller. The pressurized fluid then exits the casing via an outlet located along the periphery of the casing. Connected to the casing is an adaptor and a bearing frame that supports the rotary shaft extending therethrough. Between the casing and bearing frame is a seal chamber.
Fluid which is pumped through the casing must be prevented from flowing along the rotating shaft and leaking out of the seal chamber and into the environment or into the bearing frame. Rotating mechanical seals or rotary seals located in the seal chamber are generally used to provide this sealing. The typical rotary seal has a rotating seat circumscribing and attached to the rotary shaft and a stationary seat element attached to the seal chamber. The rotating seat may also be mounted on a shaft sleeve which is mounted about the shaft. The rotating seal includes a sealing ring having a sealing face which mates against an opposing face of the seat. The opposing sealing face and seat face form a sealing interface.
As the seal face rotates, friction is generated at the sealing interface. To reduce this friction, a lubricant must be supplied to the sealing interface. Rotary seals are designed such that a minute amount of the fluid within the seal chamber will migrate into the sealing interface to act as a lubricant. This lubricating fluid forms what is referred to as the interface film.
As can be appreciated, the rotation of the exterior surface of the shaft and rotating seal create flows within the fluid located within the seal chamber. In many pump applications, for the rotary seal to continue to function properly, this flow within the seal chamber must perform several functions. The flowing fluid must function to remove heat from the rotary seal, prevent the collection of vapor and gas bubbles around the sealing rings, and prevent solids which are suspended within the fluid from migrating into the sealing interface. However, the fluid flow created within the seal chamber by the rotating shaft and seal is frequently inadequate to perform the above-mentioned functions.
Removing heat from the rotary seal is often the most important functional requirement of the fluid flow in the seal chamber. Even though friction at the sealing interface is reduced by the interface film, the rotary seal can generate considerable heat. If the temperature of the rotary seal becomes elevated due to inadequate heat removal, the likelihood of the interface film vaporizing increases. This vaporization would remove the lubrication between the sealing surfaces causing seal instability and may distress the seal face leading to seal failure. Therefore, the flowing fluid must remove this heat through convection and transfer the heat to a heat sink such as the pump body or the fluid flowing through the casing. Thus, there is a need to direct fluid flow about the seal to improve the heat transfer and prevent a harmful rise in the temperature of the rotary seal and sealing rings.
Fluid flow within the seal chamber must also remove vapor or gas bubbles which can also lead to problems with the rotary seal. There are usually two main sources of vapor in the seal chamber. First vapor can originate from the seal interface if interface film vaporization is occurring. Secondly, many of the fluids pumped may have a trapped gas or vapor. Due to the typical fluid flow dynamics within the seal chamber, much of the vapor within the chamber will be centrifuged toward the pump shaft and into recesses along the seal. The bubbles become trapped by these recesses. Ultimately, the bubbles link up to form a continuous toroid which effectively isolates the sealing interface from the cooling liquid and causes the seal faces to run hotter, causing complete interface film vaporization. A seal under such conditions stands little chance of survival. Directing the fluid in the seal chamber to flow along the rotary seal will flush away the entrapped bubbles. There is a need to direct the fluid flow within the seal chamber to prevent the collection of bubbles about the rotary seal.
The fluid flow in the seal chamber must also flush out solid particles suspended within the pumped fluid as problems can also occur with the rotary seal when the suspended solids flow into the seal chamber. These solids collect around the seal and penetrate the sealing interface where they may become embedded and also can concentrate in the rear of the seal chamber causing wear on the seal chamber. The collection of solids around the seal may break up the interface film and cause extra heating and wear, leading to premature seal failure. Although directing the fluid to flow along the rotary seal will flush away the collected solids, it has been found that directing a flow inward along the rotating shaft and rotary seal and outward along the periphery of the seal chamber is best suited for ejecting the suspended and collected solids. Thus, there is a need to direct the fluid flow within the seal chamber to eject particles away from the sealing rings and sealing interface.
Therefore, there has been a need to direct the flow of fluid within the seal chamber to allow the fluid to remove heat from the rotary seal, remove bubbles from recesses in the seal, and flush solid particles away from the sealing rings and interface.
In an attempt to prevent solid particles from reaching the mechanical seal, a protector device, as disclosed in U.S. Pat. No. 4,872,690, employs a separate annular cup-shaped element which is secured to the pump housing at the entrance to the seal cavity. A circular opening in the cup-shaped element of the protector allows the rotary shaft to pass through the protector with a small annular clearance between the shaft and element. The cup-shaped element also includes a plurality of vent passages around the element's outer circumference. As the shaft and impeller rotate, fluid passing by the vent passages creates a low pressure area outside the seal chamber, thus creating a looped fluid flow around the protector. Fluid, having suspended solids, enters the seal chamber through the annular clearance and is drawn out through the vent passages before the solids are deposited within the chamber.
This protector device is ineffective in correcting a number of the problems described above as well as other problems found in the operation and maintenance of these centrifugal pump systems. One problem is that the small annular clearance and vent passages may become clogged with the solid particles reducing the effectiveness of the protector device. Also, the small annular clearance between the protector device and the shaft results in a wear zone on the impeller due to local increased particle velocity.
An additional problem with the use of the protector is that rotary seals are mounted about the shaft and therefore the diameter of the shaft and mounted seals will be greater than that of the shaft at the point adjacent the cup-shaped element. Thus the small annular clearance between the circular opening of the cup-shaped member and the rotary shaft prevents removal of the seal mounted rotary shaft which is necessary to service the rotary seal. Therefore, to perform maintenance on the rotary seal, the protector must be removed. This lengthens the time and effort required to replace or repair the mechanical seals.
In summary, there is a need for a device which directs the fluid flow within the seal chamber to increase the rate of heat transfer from the rotary seal and flush bubbles or particulates away from the seal interface.